For a conventional single-stage AC-DC converter with high power factor, the output voltage usually contains a low frequency (twice line frequency) ripple component, due to imbalance of input and output power. This second harmonic (e.g., 120 Hz in North America or 100 Hz in China,. Europe) is of particular concern for DC lighting applications, such as LED lighting, as it results in visible flickering wherein the human eye can see fluctuation of the light emitting from the LED.
FIG. 1 shows a conventional single-stage flyback converter, typically used in medium to low power applications such as driving LEDs. The low frequency ripple of the output voltage is usually significant and results in a ripple in the LED current, which produces fluctuating light output, which may be undesirable in certain lighting applications, as well as harmful to human eyes.
In order to solve this problem, a two-stage AC-DC converter may be used to produce ripple-free DC output while maintaining a high power factor at the AC side. FIG. 2 shows such a two-stage AC-DC converter used to drive LEDs. The first power stage is a boost converter used to achieve a high power factor. The output voltage of the Boost converter is usually high, e.g., 380 VDC, which must be lowered to drive most loads. The second stage is a DC-DC converter used to step down output voltage and produce the final DC output. The DC-DC converter can be implemented with different topologies such as buck, flyback, LLC, etc. In some cases, the first stage power factor correction can also be implemented with a flyback converter as shown in FIG. 3. However, in general, the addition of the second stage has drawbacks such as reduced efficiency and higher component count and associated cost.